In optical communications networks, transceivers are used to transmit and receive optical signals over optical fibers. A laser of the transceiver generates amplitude modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. FIG. 1 illustrates a block diagram of a known transceiver currently used in optical communications, which uses optical feedback to control the average output power level of the laser. The transceiver module 2 includes a transmitter portion 3 and a receiver portion 4. The transmitter and receiver portions 3 and 4 are controlled by a transceiver controller 6. The transmitter portion 3 includes a laser driver 11 and a laser diode 12. The laser driver 11 outputs electrical signals to the laser diode 12 to modulate the laser diode 12 to cause it to output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system (not shown) of the transceiver module 2 focuses the coherent light beams produced by the laser diode 12 into the end of a transmit optical fiber (not shown).
A low-speed monitor photodiode 14 monitors the output power levels of the laser diode 12 and produces respective electrical analog feedback signals that are delivered to an analog-to-digital converter (ADC) 15, which converts the electrical analog signals into electrical digital signals. The digital signals are input to the transceiver controller 6, which processes them to obtain the average output power level of the laser diode 12. The controller 6 outputs control signals to the laser driver 11 to cause it to adjust the bias current signal output to the laser diode 12 such that the average output power level of the laser diode 12 is maintained at a relatively constant level.
The receiver portion 4 includes a receive photodiode 21 that receives an incoming optical signal output from the end of a receive optical fiber (not shown). An optics system (not shown) of the receiver portion 4 focuses the light output from the end of the receive optical fiber onto the receive photodiode 21. The receive photodiode 21 converts the incoming optical signal into an electrical analog signal. An ADC 22 converts the electrical analog signal into an electrical digital signal suitable for processing by the transceiver controller 6. The transceiver controller 6 processes the digital signals to recover the data represented by the signals.
At times, it is desirable to obtain measurements relating to the optical signals produced by the laser in addition to the average output power level of the laser. For example, tests are commonly performed in networks to measure jitter performance. With respect to optical signals generated by lasers, jitter performance relates to variations in the timing at which the optical signal output from the laser transitions from a logic 1 power level to a logic 0 power level (falling edge) or from a logic 0 power level to a logic 1 power level (rising edge). Jitter is essentially a slight variation in the phase of the optical signal from that of a corresponding ideal waveform. If there is too much jitter in the signal produced by the laser of the transceiver, a receiver that receives the optical signal might detect a logic 1 power level when it should have detected a logic 0 power level, and vice versa.
Jitter performance is usually measured by using test equipment that is inserted into the network. Inserting the test equipment into the network requires that the network be taken down, which is time consuming and burdensome. Likewise, removing the equipment after the measurements have been obtained and putting the network back up is also time consuming and burdensome. In addition, communications are disrupted during the entire process from the time the network is taken down until it is put back up, which of course is undesirable.
It would desirable to provide a way to measure jitter performance that does not require the insertion of equipment into and removal of equipment from the network. It would also be desirable to provide a way to obtain these measurements in real-time with no interruption in communications over the network.